This background presents the various means that have used to deal with the whisker problem. Only a few means prevent whisker risk. (Not knowing of any prevention means, advocates for the rest (discussed below) have acknowledged that their means only mitigate.)
Since the beginning of electronic product manufacturing, tin-lead (Sn—Pb) solder (commonly the eutectic alloy) has been used virtually exclusively to make the interconnections between the electronic components (e.g., transistors, sockets, resistors, diodes, capacitors, integrated circuits, and the like) that comprise the assembly. This solder has a convenient melting temperature and provides reliable and reparable connections. More recently, a Sn-based solder containing small amounts of silver (Ag) and copper (Cu), but no Pb, has become widely used.
Fabricated Boards
When electronic assembly design switched from vacuum tubes to “solid state” design, in place of terminals and wires, an insulating “bare” (i.e., unpopulated) circuit board with etched cooper “lands” and “traces” was introduced, and continues in use today. It provides a mounting platform for the components, with the lands as the solder-connection sites for the component terminations, and the traces making the interconnections between the lands. (The industry refers to making a bare board as “fabrication.”)
At first, component terminations were wires inserted through “vias,” holes through the board surrounded by lands at the ends of the copper traces that connect between components. Later, as the size, and spacing between, component terminations diminished, through-hole wires were replaced by much smaller terminations—connection points that, of soldering, not only made the intended electrical connections to the lands but also attached the component to the board. This construction is referred to as “surface mounting”.
Finish
Since uncoated copper does not solder well with the soldering fluxes permitted for electronic assemblies, a tin-lead alloy was typically applied over the copper as the “finish” (i.e., the final layer). The finish quickly develops a very thin protective oxide coating, and by protecting the underlying basis metal from oxidation, preserves the “solderability” ability to be wet by solder) of the lands. For the same reason, a solderable finish is also applied over the basis metal of the component terminations (typically a copper alloy or alloy 42).
Until enactment of legislation prohibiting Pb (discussed below), the most widely used finish for boards and component terminations, was a tin-lead alloy. Today, “Pb-free Sn” (the legislation requires that it contain not more than 0.1 percent Pb by weight) is widely used as a finish. As discussed below, this use creates a significant risk that long after the product has entered the field, short circuits will develop by the growth from the Sn of “whiskers.”
Boards
Boards are designed and fabricated to customer specification. A number of Pb-free solderable finishes other than Sn are available, so even if an assembler is required to build a Pb-free assembly, a board finished with any one of these options has no whisker risk.
Components
In contrast, components are produced by component manufacturers to their own design and specification. Typically, for a given component, a manufacturer offers only one termination finish, which for about 85 percent of the components is Pb-free Sn.
Assembly and Soldering
By far the most common process to produce an electronic assembly (i.e., a board populated with components) is known as “surface-mount reflow”:                (1) Apply to each land of the board a controlled amount of solder paste (comprised of tiny solder balls, flux, and other ingredients),        (2) Position each component on its intended site, its terminations aligned with the lands and contacting the solder paste,        (3) Heat the assembly above the solder's melting temperature. The solder reflows and wets lands and terminations to form the intended connections.        (4) After cooling, some assemblies are then cleaned to remove residues; if a so-called “no-clean” flux is used this step may be omitted.        
A number of terms other than “electronic assembly” are in common use, including “circuit assembly,” “circuit card,” and just “card.” Because of ambiguity, the term “board” to refer to a populated (rather than bare) board should not be used.
Pb Prohibition and its Consequences
Out of a stated public-health concern for the toxicity of lead (Pb), 2003 certain government entities enacted legislation that has the effect of prohibiting the use of Pb both in solder and the termination finish of boards and components. (The prohibition applies directly not to these items, but to most categories of electronic equipment put on the market.) As a result, the industry has undergone a revolution in materials and processes for assembling Pb-free products.
At customer request, some board fabricators began to use a Pb-free Sn or high-Sn alloy. Concurrently, as customer demand for lead-free components grew, most component manufacturers, for cost reasons, chose (among other options) to offer Pb-free Sn as the only available finish. (Inventorying more than one finish is too expensive.)
To comply with the Pb prohibition, assemblers of equipment covered by the legislation had to switch to Pb-free solders. Regrettably, after decades of searching, no drop-in Pb-free replacement solder has been found that.                Melts at a suitable temperature, and        Does not contain Sn as a major constituent (which, had one been found, would have avoided the whisker risk).        
Even more regrettably, because customers have no choice of component termination finish, even those building equipment not covered by the Pb prohibition find themselves obliged to buy the components they need with a Pb-free Sn termination finish that, because of its whisker risk, they do not want.
These users have the option, of course, to replace the termination finish with conventional tin-lead solder. But this is an expensive proposition, and not without risk:                Small chip components can have the finish replaced by a proprietary process from a single source.        Larger components can have the terminations dipped in solder one side at a time, with the associated (unquantifiable) risks of undetectable damage from the sudden asymmetric heating from dipping the terminations of just one side of the component at a time in solder, or from handling.Unless contractually required to, most assembly manufacturers today do not replace the termination finish.        
Pb-Free Tin: The Extent of the Whisker Problem
While originally Pb was added to the Sn to give a convenient melting temperature, it was later found to have another important attribute. After over a half-century of diligent searching, it has proved to be the only substance that prevents the (often very slow) growth from the Sn of (sometimes very long) filaments, universally referred to as “whiskers.”
The mechanism by which Pb prevents whisker growth remains poorly understood, but the Pb must be “in” the Sn (i.e., alloyed with it). Remarkably, Sn whiskers penetrate a 1-μm cap of Pb in days (Ed Li, AEM, Inc., San Diego, Calif., private communication, 2006).
Hence for practical purposes, except where the bill of materials for an electronic assembly has been thoroughly “scrubbed.” by experts (and all Pb-free finishes replaced), and the termination composition of every component checked upon receipt, manufacturers of all kinds of electronic assemblies (including those not covered by the prohibition of Pb) now must assume that the assemblies they are building contain Pb-free Sn plating.
In the same way, every purchaser of an electronic assembly must make the same assumption, and that it may also contain Pb-free Sn solder. Consequently, virtually all electronic equipment is at risk of developing Sn whiskers. The longer the intended life of the equipment, the greater the risk that it will fail due to whisker growth.
Sn Whiskers: Symptoms and Etiology
A typical Sn whisker has a thickness in the tens of micro-inches (i.e., tens of millionths of an inch) and may grow to a length of tenths of an inch (a whisker length of one inch has been documented). Since a typical gap between individual connections in an electronic circuit (such as those on an integrated circuit package) is but a few hundredths of an inch, a whisker can easily grow long enough to cause a short circuit.
Further, while such whiskers are thin, the presence of just one on a circuit capable of supplying enough current may initiate an arc (plasma) that destroys the entire assembly.
Just as the mechanism by which Pb in Sn prevents the growth of Sn whiskers remains mysterious, so too are the causes of growth of whiskers from Pb-free Sn surfaces. Such whiskers will grow in any environment, e.g., with or without gravity or an electric field, over a broad range of temperatures and humidity, in air or vacuum, etc.
Whisker growth from apparently identical specimens may vary widely and unpredictably, in times to onset of formation, growth rates, number of whiskers per unit area distribution of lengths and thicknesses, etc. Whisker-caused short circuits have occurred in some cases after only days or months of manufacture, and in others only after more than a decade.
At the present level of understanding, it appears that the growth of whiskers has many contributing causes. Sn plated directly onto copper reacts with it (by solid-state diffusion) to torn intermetallic compound (IMC) that puts it in compression. It is often claimed that whiskers grow to relieve that stress. However, whiskers also grow from Sn on substrate that do not react with Sn to form an IMC, so even to the extent that explanation is true, it is hardly sufficient.
Because of the many conditions that may result in whisker growth, not all known, and none of which is practical to inspect for, it must be assumed that it will be a long time before users can rest assured that the Sn on all the components they receive will not grow whiskers. Also, the prospects seem dim for any non-capping process that would nullify Sn's whiskering proclivity.
How Many System Failures are Due to Sn Whiskers?
The frequency of occurrence of whisker-caused short circuits remains poorly documented.                A short-circuit event often obliterates all evidence of the whisker that caused it, making it difficult to diagnose.        Because whiskers often grow slowly, any scientific investigation is very time-consuming.        Due to their microscopic thickness, whiskers are difficult for trouble-shooters to detect visually.        Most importantly, manufacturers whose products have been whisker victims are loathe to admit it publicly, lest they face demands for a recall.        
As a result, the risk that a give assembly will fail due to a whisker-caused short circuit has proved impossible to quantify.
Mitigation on Boards and Components
While the current invention involves plating, it differs qualitatively from the prior art of plating bare boards and component terminations. Applicants, who each have decades of experience in electronics manufacturing, could find no prior art of applying plating to an electronic assembly, for the disclosed purpose, or for any other. Given the magnitude of the whisker problem, had the practice of capping Pb-free Sn with an impenetrable metal been used, it would have become well-known within the industry.
As for plating components and bare boards, there are many cases of prior art, but none that relate to the present invention. For example, U.S. Pat. No. 5,882,736 teaches the deposition of palladium onto copper substrates of unpopulated circuit boards, to achieve improved wire bonding of components subsequently attached to the circuit board, to populate it.
(Wire bondability is rarely a concern for bare boards, since most connections are made with solder, not by bonding wires. Where it is a requirement, palladium is generally used due to its lower cost compared to other noble metals.)
The '736 patent asserts that the resulting layer of palladium on the copper substrate limits the formation of the intermetallic compound of Sn and copper (Cu) that would otherwise form when Sn is plated (or soldered) directly onto copper.
Various baths for the chemical deposition of metal layers are known in the art (e.g., for palladium, U.S. Pat. No. 4,424,241, U.S. Pat. No. 3,418,143, U.S. Pat. No. 3,754,939, DE-OS 42 01 129, GB-PS 1,164,776, DE-OS 30 00 526, U.S. Pat. No. 4,341,846, U.S. Pat. No. 4,255,194, DE-OS 28 41 584, and EP-0 423 005 A1). For many metals, electroless plating bath chemicals are commercially available. (The Appendix shows chemical reactions believed to occur in an electroless nickel plating bath.)
Mitigation by Underplating Component Termination Finish
To counter the whisker risk from Pb-free Sn, some component manufacturers pre-plate onto the termination basis metal (usually Cu or alloy 42) a layer of another metal, most commonly nickel (Ni), as a diffusion barrier before the Sn finish is applied. (See for example “Understanding Whisker Phenomenon: Driving Force for Whisker Formation” by (Then Xu, Yun Zhang, C. Fan and J. Abys.)
However, depending on the barrier's characteristics, this approach has reduced whisker growth in some situations, and enhanced it in others. Termination forming after plating may crack a brittle Ni underlayer, resulting in loss of its barrier property and actually promoting whisker growth.
Just as the component purchaser has no practical way to assess the quality of the Sn, he has no practical way to evaluate the underplating, and hence no way to know whether it mitigates or aggravates whisker risky. No one claims that it eliminates whisker risk.
Mitigation by Polymer-Coating Assemblies
Assembly manufacturers have had few whisker-mitigation options applicable to entire soldered electronic assemblies. If a manufacturing process, applied to an assembly, were able to prevent, and not just mitigate, whisker risk, it would obviate specialists' “scrubbing” the bill of material and inspecting for Pb-free Sn on incoming components, a practice widely found in high-reliability electronics manufacturing today.
The simplest and least expensive risk-mitigation technique has been to depend on the conformal polymer coating that is widely applied to the assembly anyway (especially on high-reliability assemblies intended for long service life). A conformal coating is applied to an electronic assembly designed to operate in a humid environment to reduce the risk of corrosion and electrochemical migration (dendritic growth), which can result in a short circuit.
(Dendrites and whiskers are entirely separate phenomena. Dendrites grow in the presence of moisture and a DC electric field on an insulating surface between uncoated conductors. Whiskers grow, in any direction, out of the surface of a metal subject to whiskering, with no moisture or field required.)
However, commercially available conformal coating materials are formulated to prevent dendritic growth, not to resist penetration by whiskers. In fact, whiskers have been found to penetrate some conformal coatings within months. Even the most whisker-resistant coating formulations, applied, as specified, to a “flat, unencumbered surface” 1-2 thousandths of an inch thick, are penetrated within a few years.
An added difficulty is that the coating achieved by spraying, the most widely used application method, is much thinner, or even absent, on shadowed surfaces (e.g., the back side of terminations) and (due to surface tension effects) along the edges of rectangular component terminations.
Acrylics, the easiest of the conformal coatings to apply and remove for rework or repair), are also among the most easily penetrated. Thus, an engineer choosing to use conformal coating for the additional function of mitigating whisker risk may have to compromise between manufacturing convenience and penetration resistance.
The mechanical properties of all polymers vary (reversibly for many) with temperature and humidity. This includes the relevant properties of those used for conformal coating (adhesion to the substrate, pliability vs. brittleness, whisker penetration resistance, etc.). Also, beyond certain limits, temperature, humidity, and age can cause irreversible deterioration that lessens a coating's whisker penetration resistance. A coating's variability and susceptibility to deterioration are rarely investigated, by supplier or user.
Parylene™, applied in a vacuum chamber by a process that ensures total coverage, has been found to be among the most whisker-resistant coatings. (It has the disadvantage that it cannot be removed by a solvent for rework and repair.) Even Parylene coatings have been penetrated within a few years, and that is with test specimens held at or near room temperature. Hence, commercially available conformal coatings are at best whisker-resistant, not whisker-impenetrable—risk mitigators, not eliminators.
Despite the shortcomings of polymer conformal coatings, they do mitigate whisker risk. For a penetrating whisker to actually cause a short circuit, one of two events each fairly unlikely if the conformal coating coverage is complete) must occur:                It must meet another whisker from a surface at a different electrical potential that has also penetrated the coating, OR        It must penetrate the conformal coating over a metal at a different electrical potential, not from below, but from the outside.        
Prevention by Polymer-Coating Assemblies
A branch of the US Missile Defense Agency has been funding efforts to formulate a polymer conformal coating, known as “Whisker-tough”™, expressly for penetration resistance. The impenetrability results, not from its rigidity, but from its resilience and tailored adhesion strength. By design, an advancing whisker lifts, or “tents,” the coating away from the surrounding Sn surface until it gets so long that the resistance force from the coating buckles it and re-directs it back towards the surface.
Application is simple (although it does differ from the most widely used method—spraying). An assembly is immersed in the liquid “Whisker-tough”™ coating material, and the excess is allowed to drain off. The material includes a thixotropic agent to ensure edge coverage. The coating is air-cured first at room temperature and then in an oven. MDA hopes that the formulation will serve as a drop-in replacement for conventional conformal coatings, including ease of application, so that it can permit the use of Pb-free Sn finishes and solder in the systems it buys.
However, it, like other polymer systems, is subject to (reversible or irreversible) changes in its properties at temperature and humidity extremes, and irreversible changes with age. The influence of these variables has yet to be investigated. (Steve Smith, Whisker-Tough LLC private communication, 2011.)
Prevention by Ceramic-Coating Assemblies
A vacuum process for depositing a (non-polymer) very thin whisker-impenetrable ceramic cap on all assembly surfaces has become available from Sundew Technologies. Although Sundew's atomic layer deposition process has been used for a number of years in electronic chip device fabrication, its use for producing a whisker-impenetrable cap is recent.
The cap's composition can be to within limits, to give desired physical properties (e.g., resilience, yield stress). If undamaged during handling, the cap is virtually unaffected by temperature, humidity and age. Such coatings have been shown to prevent whisker penetration for more than one year.
Surface preparation to ensure good adhesion of the ceramic film to the many different materials present in an assembly (at least all the Pb-free Sn and Sn rich alloys) is a requirement of unknown difficulty. (For most assemblies, adhesion to surfaces other than Sn may not be important. Any film that flaked off from them would not impair performance or reliability, and being so thin, might not even be noticed.)
The size of the cap's thickness safety margin is not yet known. A coating too thin would be punctured by a Sn whisker or damaged during handling, while even with adequate surface preparation, due to expansion coefficient differences, a coating too thick would fracture during temperature cycling.